1. Field of the Invention
This invention relates generally to integrated circuit fabrication and, more particularly, to masking techniques.
2. Description of the Related Art
As a consequence of many factors, including demand for increased portability, computing power, memory capacity and energy efficiency in modern electronics, integrated circuits are continuously being reduced in size. To facilitate this size reduction, the sizes of the constituent features, such as electrical devices and interconnect line widths, that form the integrated circuits are also constantly being decreased.
The trend of decreasing feature size is evident, for example, in memory circuits or devices such as dynamic random access memories (DRAMs), static random access memories (SRAMs), ferroelectric (FE) memories, etc. To take one example, DRAM typically comprises millions of identical circuit elements, known as memory cells. In its most general form, a memory cell typically consists of two electrical devices: a storage capacitor and an access field effect transistor. Each memory cell is an addressable location that can store one bit (binary digit) of data. A bit can be written to a cell through the transistor and read by sensing charge on the storage electrode from the reference electrode side. By decreasing the sizes of constituent electrical devices and the conducting lines that access then, the sizes of the memory devices incorporating these features can be decreased. Additionally, storage capacities can be increased by fitting more memory cells into the memory devices.
The continual reduction in feature sizes places ever greater demands on techniques used to form the features. For example, photolithography is commonly used to pattern features, such as conductive lines, on a substrate. The concept of pitch can be used to describe the size of these features. Pitch is defined as the distance between an identical point in two neighboring features. These features are typically defined by spacings between adjacent features, which are typically filled by a material, such as an insulator. As a result, pitch can be viewed as the sum of the width of a feature and of the width of the space separating that feature from a neighboring feature. Due to factors such as optics and light or radiation wavelength, however, photolithography techniques each have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, the minimum pitch of a photolithographic technique can limit feature size reduction.
Pitch doubling is one method proposed for extending the capabilities of photolithographic techniques beyond their minimum pitch. Such a method is illustrated in FIGS. 1A-1F and described in U.S. Pat. No. 5,328,810, issued to Lowrey et al., the entire disclosure of which is incorporated herein by reference. With reference to FIG. 1A, photolithography is first used to form a pattern of lines 10 in a photoresist layer overlying a layer 20 of an expendable material and a substrate 30. As shown in FIG. 1B, the pattern is then transferred by an etch step (preferably anisotropic) to the layer 20, forming placeholders, or mandrels, 40. The photoresist lines 10 can be stripped and the mandrels 40 can be isotropically etched to increase the distance between neighboring mandrels 40, as shown in FIG. 1C. A layer 50 of material is subsequently deposited over the mandrels 40, as shown in FIG. 1D. Spacers 60, i.e., material extending or originally formed extending from sidewalls of another material, are then formed on the sides of the mandrels 40 by preferentially etching the spacer material from the horizontal surfaces 70 and 80 in a directional spacer etch, as shown in FIG. 1E. The remaining mandrels 40 are then removed, leaving behind only the spacers 60, which together act as a mask for patterning, as shown in FIG. 1F. Thus, where a given pitch formerly included a pattern defining one feature and one space, the same width now includes two features and two spaces defined by the spacers 60. As a result, the smallest feature size possible with a photolithographic technique is effectively decreased.
It will be appreciated that while the pitch is actually halved in the example above, this reduction in pitch is conventionally referred to as pitch “doubling,” or, more generally, pitch “multiplication.” That is, conventionally “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor. The conventional terminology is retained herein.
Because the layer 50 of spacer material typically has a single thickness 90 (see FIGS. 1D and 1E) and because the sizes of the features formed by the spacers 60 usually corresponds to that thickness 90, pitch doubling typically produces features of only one width. Circuits, however, often employ features of different sizes. For example, random access memory circuits typically contain arrays of memory cells and logic circuits in the so-called “periphery.” In the arrays, the memory cells are typically connected by conductive lines and, in the periphery, the conductive lines typically contact landing pads for connecting arrays to logic. Peripheral features such as landing pads, however, can be larger than the conductive lines. In addition, periphery electrical devices such as transistors can be larger than electrical devices in the array. Moreover, even if peripheral features can be formed with the same pitch as the array, the flexibility required to define circuits will typically not be possible using a single mask, particularly if the patterns are limited to those that can be formed along the sidewalls of patterned photoresist.
Some proposed methods for forming patterns at the periphery and at the array involve etching a pattern into the array region of a substrate and into periphery of the substrate separately. Thus, a pattern in the array is first formed and transferred to the substrate using one mask and then another pattern in the periphery is formed and separately transferred to the substrate using another mask. Because such methods form patterns using different masks at different locations on a substrate, they are limited in their ability to form features that require overlapping patterns, such as when a landing pad overlaps an interconnect line, and yet a third mask may be necessitated to “stitch” two separate patterns with interconnects. Additionally, such a third mask would face even greater challenges with respect to mask alignment due to the fine features defined by the pitch multiplication technique.
Accordingly, there is a need for methods of forming features of different sizes, especially where the features require different overlapping patterns and especially in conjunction with pitch multiplication.